In recent years we have provided the first insight into the molecular structure of cryptochrome, a flavin-containing blue light photoreceptor. We first isolated cryptochrome from the plant Arabidopsis - however, the photoreceptor appears to be present in all higher plants and is also found in ferns and algae. Recently, cryptochrome-like genes have been described for mammals, including humans. In plants, cryptochrome plays an important role in the perception of light in the surrounding environment - plants adjust their growth in response to light- induced signaling mediated by this photoreceptor. It seems likely that the human cryptochrome-like protein mediates entrainment of circadian behavioral rhythms. The long-term objectives of this proposal are to define the molecular mechanism by which cryptochrome perceives and transmits a light signal. We will use both a molecular approach and the power of Arabidopsis genetics to carry out these studies.